1. Technical Field
The present invention relates to magnetoresistive transducers. More particularly, the present invention relates to a class of materials used as a dielectric to separate various metallic layers within a magnetoresistive transducer.
2. Description of the Prior Art
Magnetoresistive (MR) transducers for sensing a magnetic flux produced by a moving medium are known. A dual stripe MR transducer is described in U.S. Pat. No. 3,860,965, issued to Voegeli. Such dual stripe MR transducer design is currently considered state of the art because it produces the largest output per unit track width, has a low noise level due to common mode rejection, and provides linear and symmetric off-track response.
In a transducer constructed in accordance with such design, two closely spaced MR stripes are sensed differentially. The close spacing of the MR stripes is desirable as it allows greater storage capacity on a magnetic medium that is to be read by the transducer. In addition, the proximity of the two stripes is required so that the proper biasing condition can be established without exceeding the current density limit of the magnetoresistive stripes.
If the dual stripe MR design is to function properly the two MR stripes must be electrically isolated from one another, at least at one end of the pair. Otherwise, the stripes would be shorted together, thereby rendering the transducer inoperable.
In some MR transducer designs it is also useful to employ equally small distances between the MR stripes and adjacent ferromagnetic shields. Again, to produce a useful MR transducer, the MR stripes must be close to the shields and yet remain electrically isolated from the shields.
FIG. 1 shows a perspective view of a dual stripe MR transducer of the type disclosed in the '965 patent. In FIG. 1, a first MR device is shown, consisting of conductive leads 10,11 which are spaced apart along an axis of the first MR stripe 14 to provide a gap 15. The gap defines an active area corresponding to the read track width of the media to be read by the transducer.
In dual stripe MR transducer designs, a second device having conductive leads 12,13 spaced apart along an axis of the second MR stripe 18 to form a gap 15a is provided in parallel with the first element.
FIG. 2 is a cross-sectional view of a dual stripe MR transducer showing the active area of the transducer. Thus, portions of the two MR stripes 14,18 are shown in a parallel spaced relation to one another. The spacing 16 between the stripes is maintained by an interMR dielectric 20. Additional dielectrics 22,24 space the MR stripes from ferromagnetic shields 25,26. An air bearing surface 27 is provided by which the transducer floats over a magnetic storage medium that is being read by the transducer.
In operation, current is introduced into the conductors in the direction indicated by the arrows i.sub.1,i.sub.2. These currents are propagated along the MR stripes and serve as both sense and bias currents. An instantaneous value of the magnetic flux from the portion of the medium which is moving proximate to the gap 15 at any given time is measured by the dual stripe transducer. Such flux value corresponds to bits of data stored on the medium.
While the '965 patent teaches a 30 nm separation between the MR stripes, there is no indication that such device was actually fabricated. In fact, maintaining electrical isolation across such small dielectric thicknesses has been a major impediment to the implementation of the dual stripe MR design. It has been the experience of researchers in the relevant art that reliably and reproducibly fabricating dual stripe MR heads with spacings of less than about 70 nm is exceedingly difficult. This is due to shorting between the two MR stripes, primarily as a result of process and material limitations.
All of the teachings to date concerning dual stripe MR heads pertain to the use of sputtered Al.sub.2 O.sub.3, SiO.sub.2, or SiO as the inter-MR dielectric. Nonetheless, in practice these materials provide unacceptable isolation between the metallic layers within the MR head for dielectric thicknesses less than about 70 nm.
There is also some teaching in the semiconductor art to perform post-deposition processing of a thin film to improve the film dielectric properties. In such cases, gate dielectrics of SiO.sub.x N.sub.y have been fabricated by exposing 5-10 nm SiO.sub.2 films to nitrogen ambients at elevated temperatures for short periods of time. See, for example, Process dependence of breakdown field in thermally nitrided silicon dioxide, Ramesh et al, J. Appl. Phys. 70 (4), 1991.
It has also been proposed to use Ta oxide as a gate dielectric in 64 Mbit DRAMs. See, for example, THERMCO INTERNATIONAL LABORATORY, DR212.06, Japanese Daily Report, Nov. 12, 1991. In addition, it has been taught that Ta.sub.2 O.sub.5 can be used as a capacitive dielectric in integrated circuit structures. See, for example, Highly Stable Tantalum Thin Film CR circuit on a Single Substrate, Yamazaki et al, FUJITSU Scientific and Technical Journal, December 1970. Such teachings are not particularly useful, by way of analogy, to the fabrication of MR stripes, which are generally formed of an NiFe film.
Thus, the full advantage of dual stripe MR transducer technology, i.e. extremely dense data storage on a magnetic medium, cannot be realized unless MR stripe spacings can be minimized. This, in turn, requires a new class of dielectric.